Patent GB-1354833-A (GB)
Title: Telephone instrument suitable for use in a closed-loop system
IPC: F23Q3/00
Filed: 1972-07-17
Abstract: 1354833 Telephone systems COMPAGNIE INDUSTRIELLE DES TELECOMMUNICATIONS CIT-ALCATEL 17 July 1972 [16 July 1971] 33335/72 Heading H4K A subscriber's instrument for use in a unidirectional loop communication system through which a synch. pulse is transmitted once per frame in order to enable a local clock at each instrument, comprises a time slot counter triggered by the clock and three coincidence circuits for producing outputs when the counter reaches a state indicative of i.e. the time slot permanently allocated to, a, the local station, b, the local station when this is designated as a called station by a remote station i.e. a called condition and c, a remote station when this is designated as a called station by the local station i.e. a calling condition. The system utilizes PCM or as particularly described compressed delta modulation. A subscriber A's instrument includes a pulse amplifier 10 connected in series with a loop line 2, a gate 42 for preventing retransmission of pulses arising at that instrument, a gate 41 for disabling the instrument in a calling mode if the wanted party is already busy and a pulse polarity discriminator 12 for distinguishing between positive speech pulses 13 and negative signal 15 or synch. 16 pulses. The latter are generated by a central source and comprise 0À26 Ásec. pulses having a pulse repetition rate of 30 KHz. This rate, t 0 , is chosen such that for a given length of line loop having a transmission delay time of T Ásecs., mt o  = nT where m and n are integers. The time frame contains 128 time slots (including one for the synch. pulse) of which 125 are allocated uniquely to the maximum of 125 subscribers of the system. An incoming synch, pulse is gated into frequency multiplier 20 whose output triggers a time slot counter 21. The pulse positions around that of the synch, pulse are not used by subscribers but are written in a register 22 so that the synch. pulse passing gate 14d is opened and the signal pulse passing gate 14c is closed at these times. In addition register 22 contains its own station identity so that when the counter reaches this station's time slot gates 26 and 36 are enabled to pass signal or speech pulses respectively. When station A is being called, gate 26 passes the incoming pulse series representing the identity of the caller B into a serial/parallel converter 27 and thence into a second coincidence register 23. Thereafter whenever the B time slot occurs gate 28 opens to enable a speech encoder 40 whose output is passed to line and thence to the calling party's station. The latter originated the call by keying the wanted party's identity via keyboard 25 into a third coincidence register 24 so that whenever the wanted party's time slot occurred the caller's identity was injected into line by a generator 32. The identity is transmitted as a 15 bit sequence of which the first bit is a start bit, the next seven are identity bits and the final seven are zero bits. When the called party replies a non-zero bit sequence is transmitted thereby so as to set the caller's flipflop 30 and thus inhibit his identity generator 33 and enable his speech encoder 34. A discriminator 37 prevents zero bit sequences from reaching a subscriber's loud-speaker 39.